The invention relates to a rechargeable electrochemical cell which is sealed from the external atmosphere and which comprises a sealed chamber containing (a) a positive electrode, the electrochemically active material of which can reversibly store and release a proton and an electron, (b) a negative electrode, the electrochemically active material of which consists of a metal combination which forms a hydride with hydrogen, and (c) an aqueous electrolyte solution having a pH of more than 7. In addition the cell may comprise a separator which electrically separates the electrodes but allows ion and gas transport. Hereinafter such a cell will be referred to as a "sealed cell". However, if desired, such a cell may be provided with a relief valve which becomes operative at a predetermined gas pressure in the cell. The invention also relates to a method of producing sealed cells.
A rechargeable, sealed cell of this type is, for example, disclosed in U.S. Pat. No. 3,874,928. For this known cell the electrochemically active material of the positive electrode may consist of nickel hydroxide, silver oxide or manganese oxide, nickel hydroxide generally being preferred for practical reasons. The electrochemically active material of the negative electrode, may, for example, consist of an intermetallic compound of lanthanum and nickel having an empirical formula LaNi.sub.5.
It is known that with hydride forming intermetallic compounds of this nature both the lanthanum and the nickel can be partially replaced by other metals such as, as regards the lanthanum for example, by calcium, thorium, titanium and rare earth metals, yttrium, and as regards the nickel for example, by copper, chromium and iron. (See, for example, United Kingdom Pat. No. 1,463,248.) If hereinafter LaNi.sub.5 and intermetallic compounds derived therefrom by substitution with other metals are mentioned then this should be understood to mean compounds which, in general, have the composition LaNi.sub.n, where n may be between 4.8 and 5.4. This indicates compounds with CaCu.sub.5 crystal structure whose existence range includes LaNi.sub.5. The expression existence range is understood to mean a range of concentrations in a continuous system of intermetallic compounds with which an identical structure can be realized with or without a heat treatment.
When constructing systems which are sealed from the external environment and which comprise hydrides of intermetallic compounds, the hydrogen equilibrium pressure above the hydride and the working temperature of the system must be taken into account. For the hydride of LaNi.sub.5 this equilibrium pressure is, at 20.degree. C., approximately 2.5 Bar. For the hydride of LaNi.sub.4 Cu this pressure is only about 0.7 Bar at 20.degree. C. and for the hydride of LaNi.sub.4 Cr about 0.31 Bar at 20.degree. C. If the electrochemical properties are acceptable, the latter materials will be preferred for producing sealed, rechargeable cells because the casing need not be as strong then.
In general the electrolyte solution consists of an aqueous solution of one or more alkali metal hydroxides, such as lithium hydroxide and potassium hydroxide. The separator may consist of a synthetic fiber (woven or non-woven), for example of polyamide or polypropylene, fiber.
The operation of a rechargeable electrochemical cell of this type differs fundamentally from a so-called nickel-cadmium battery, as a comparison of the electrochemical equations shows. With a rechargeable cell to which the invention relates, this equation is of the following form, wherein nickel hydroxide has been taken as the positive electrode material and the intermetallic compound is indicated by M: ##EQU1## For the known nickel-cadmium battery this equation may be written as: ##EQU2##
It can be seen that in the first case on charging as well as on discharging only a proton transfer takes place between the electrodes whereas the total quantity of electrolyte solution remains substantially constant. In the second case water is formed during charging which disappears again during discharging. In this cell measures must be taken to enable the storage of the formed water without this water obstructing the oxygen-gas transport between the electrodes. This requires additional space in the battery casing. On the basis of this difference in electrochemical behavior (and also for other reasons) measures taken to solve the problems inherent in the known nickel-cadmium cells cannot be applied to cells to which the invention relates. Such measures may even be superfluous in the latter cells as will be further explained below.
With sealed, rechargeable cells of the type to which the invention relates not only the hydrogen equilibrium pressure of the hydride of the intermetallic compounds, as explained above, is important but also the phenomena which occur during overcharging and over-discharging of these cells. Overcharging is in practice a risk which must be taken into account when designing cells for rechargeable batteries. Over-discharging is a phenomenon which can occur if one or more of a plurality of series-arranged cells, for example in a battery having three or more cells, is fully discharged at an earlier instant than the other cells owing to differences in capacity which are unavoidable during fabrication. The battery then continues to supply current. Both overcharging and over-discharging can, if no special provisions are made in the cells, result in the occurrence of high gas pressures and, as the case may be, in explosive gas mixtures being expelled through a valve. This causes the cell to dry out and the charge equilibrium between the electrodes is disturbed.